You are encountering either a PHP or JavaScript error during the AJAX call made by ACF to validate the fields.
To debug PHP errors put the following into your wp-config.php file
define('WP_DEBUG', true );
define('WP_DEBUG_DISPLAY', false);
define('WP_DEBUG_LOG', true);
Then attempt to save the post and then look in /wp-content/debug.log to see if there was a PHP error as well as what error and what script causes it.
Look in your JavaScript console to see if there’s a JS error.
